Top 30 Kotlin Developer interview questions to ask in 2026

The best Kotlin developer interview questions test three things: how a candidate handles null safety, whether they really understand coroutines, and how they write code when you are watching. Java trivia and syntax quizzes tell you almost nothing.

In this article, you’ll find 30 interview questions organized by skill area, along with what strong answers look like and how to use each question effectively in your hiring process.

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TL;DR

• Test for Kotlin thinking (null safety, coroutines, idiomatic code), not memorized Java.
• Run a short coding assessment before the interview so panel time goes to judgment, not basics.
• Use the 30 questions here in three buckets: core concepts, live coding, and experience.
• Score every candidate against one rubric (we call ours the Testlify Kotlin Screening Scorecard) so the bar stays the same.
• Match the question to the level: a junior explains val vs var cleanly, a senior reasons about dispatchers and architecture.

Why screen Kotlin developers with a test first?

Interviews are one of the most expensive parts of hiring and easy to fake. The need for screening becomes clearer when you look at the talent market. According to the Stack Overflow Developer Survey, Kotlin is used by about 9.4% of developers, making it a highly niche skill.

While the talent pool is substantial, candidate proficiency can vary widely. A resume may list Kotlin experience, but it does not tell you whether someone can work confidently with coroutines, null safety, or real-world application logic.

We recommend you make use of our Kotlin Developer Test to identify candidates with proven Kotlin skills before moving them to the final interview stage. That is the whole point of skills testing: spend your hours on the candidates who already proved they can code.

We built Testlify around one idea: rate every candidate on the same skills, in the same order, so a strong interviewer and a nervous one reach the right judgement call.

What skills should a Kotlin developer have?

A strong Kotlin developer is fluent in null safety, comfortable with coroutines for async work, and writes idiomatic Kotlin instead of Java with a new file extension. Beyond the language, they know your target platform, whether that is Android with Jetpack Compose or a backend service. Here is what to check and how to check it.

20 core Kotlin interview questions to ask

These cover the fundamentals every Kotlin developer should answer without thinking hard. Listen less for textbook definitions and more for whether they can say why a feature exists and when they would skip it.

1. What problem does null safety solve, and how do ?, ?., and !! differ? (A good answer flags !! as a smell.)
2. When do you use val versus var, and why default to val?
3. What is a data class, and what does it generate for you?
4. How do extension functions work, and when do they hurt readability?
5. Explain the difference between a list and a mutable list in Kotlin.
6. What are scope functions (let, run, apply, also, with), and how do you choose one?
7. How does a coroutine differ from a thread?
8. What does suspend mean on a function?
9. How do you pick a dispatcher (Main, IO, Default) and why does it matter?
10. What is structured concurrency, and what problem does it prevent?
11. How does Flow differ from a suspend function that returns a list?
12. What is the difference between a sealed class and an enum?
13. How do you handle exceptions inside coroutines?
14. What is the difference between == and === in Kotlin?
15. How do higher-order functions and lambdas work, and what is inline for?
16. What are companion objects, and when do you reach for one?
17. How does Kotlin interoperate with existing Java code?
18. What is a coroutine scope, and how does cancellation propagate?
19. When would you use a delegate (by lazy, observable) over plain code?
20. What is your rule for when a piece of logic becomes its own function or class?

How do you test Kotlin coding skills live?

Talk is cheap, so watch them code. Keep each task to 10 to 15 minutes, let them use the docs, and care more about how they reason than whether they finish. These five work well in a shared editor.

1. Write a function that safely parses a string to an Int and returns null on failure, with no try/catch leaking out.
2. Given a list of orders, group them by customer and sum the totals using Kotlin collection functions.
3. Build a small coroutine that fetches two endpoints in parallel and combines the results.
4. Turn a callback-based API into a suspend function.
5. Refactor a Java-style class they are given into idiomatic Kotlin (data class, null safety, scope functions).

Which questions reveal a developer’s experience?

Senior signal hides in tradeoffs and war stories, not definitions. A developer who has shipped Kotlin at scale will answer these with specifics: a real bug, a real decision, a real regret. Vague answers here usually mean thin experience.

1. Tell me about a concurrency bug you caused or fixed. What was the root cause?
2. Where have you used Kotlin Multiplatform, and where did you decide against it?
3. Describe a time idiomatic Kotlin made a codebase clearly better, and a time it made it worse.
4. How do you test code that uses coroutines and Flow?
5. What is a Kotlin feature you used to overuse and have since pulled back on?

When should you ask these in the hiring flow?

Put the questions where they do the most work. Use the timed coding test as a screen, the 20 core questions in the first technical interview, and the experience questions in the final round with senior engineers. This order respects everyone’s time and keeps your scoring consistent.

1. Application screen: A 20 to 30 minute Kotlin assessment, auto-scored, before any human time.
2. First interview: 6 to 8 of the 20 core questions, plus one live coding task.
3. Final round: The 5 experience questions, with a senior engineer probing tradeoffs.
4. Debrief: Every interviewer scores on the same scorecard before they hear each other’s views.

For the wider technical hiring picture, our guide to problem-solving pairs well with this list, and you can see the full library on the Testlify platform.

Hire Kotlin developers with confidence

Put these questions to work behind a real screen. Use the Kotlin Developer Test to assess null safety, coroutines, and live coding before you book a single interview, then spend your panel time on the people who already cleared the bar.

Start free or book a demo if you need help integrating the Kotlin assessment or other developer hiring tests in your hiring flow.

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Reuben

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Reuben John is a B2B content writer focused on HR and recruitment. His work explores hiring trends, skills-based recruitment, talent assessment, and the technologies shaping how companies find and hire talent.